1. Field of the Invention
The invention relates generally to audio amplifiers, and more particularly to systems and methods for minimizing startup transients in digital audio controllers that may result in audible artifacts in the output of an audio amplification system.
2. Related Art
Audio amplifiers are designed to receive input signals that typically have very low voltages and/or currents, and to generate corresponding output signals that generally have much higher voltages and/or currents. The input audio signal may undergo various types of signal conditioning and other processing which is generally intended to maintain the fidelity of the output signal with respect to the input signal. There may, however, be conditions in the operation of the amplifiers that cause unwanted artifacts in the output signal
For example, in a Class-D pulse-width modulation (PWM) amplifier, transient currents that are generated in the output stage of the amplifier at startup may produce an audible “pop.” This may result from any of several different causes, such as asymmetric power supply voltages on split rail half-bridge output stages, the charging of LC low-pass filter capacitors, or the charging of DC blocking capacitors.
In prior art systems, a typical solution to the problem of startup transients in high power outputs would be to add a relay in series with the speaker output. This relay would disconnect the speaker at startup so that the amplifier's output signal would not be applied to the speaker. As a result, none of the startup transients reach the speaker, and no associated pop is heard. After the system has settled and no significant startup transients remain, the relay is closed and the speaker is connected to the amplifier output. In prior art systems which have line-level outputs, clamp transistors are sometimes used to hold the output to a specific, benign voltage while the system initializes. After initialization, the clamp transistor is released, and the output is driven by the amplified audio signal.
While these prior art solutions to the problem of startup transients may be effective, they are not ideal. One reason for this is that components such as relays and clamp transistors which are used in the systems described above add both complexity and cost to the system. It would therefore be desirable to provide systems and methods that minimize the effects of startup transients while also avoiding the cost and complexity of adding extra components into the system.